How to process each output line in a loop?

Question

I have a number of lines retrieved from a file after running the grep command as follows:

var=`grep xyz abc.txt`

Let’s say I got 10 lines which consists of xyz as a result.

Now I need to process each line I got as a result of the grep command. How do I proceed for this?

Answer 1

One of the easy ways is not to store the output in a variable, but directly iterate over it with a while/read loop.

Something like:

grep xyz abc.txt | while read -r line ; do
    echo "Processing $line"
    # your code goes here
done

There are variations on this scheme depending on exactly what you're after.

If you need to change variables inside the loop (and have that change be visible outside of it), you can use process substitution as stated in fedorqui's answer:

while read -r line ; do
    echo "Processing $line"
    # your code goes here
done < <(grep xyz abc.txt)

Answer 2

You can do the following while read loop, that will be fed by the result of the grep command using the so called process substitution:

while IFS= read -r result
do
    #whatever with value $result
done < <(grep "xyz" abc.txt)

This way, you don't have to store the result in a variable, but directly "inject" its output to the loop.

---

Note the usage of IFS= and read -r according to the recommendations in BashFAQ/001: How can I read a file (data stream, variable) line-by-line (and/or field-by-field)?:

The -r option to read prevents backslash interpretation (usually used as a backslash newline pair, to continue over multiple lines or to escape the delimiters). Without this option, any unescaped backslashes in the input will be discarded. You should almost always use the -r option with read.

In the scenario above IFS= prevents trimming of leading and trailing whitespace. Remove it if you want this effect.

Regarding the process substitution, it is explained in the bash hackers page:

Process substitution is a form of redirection where the input or output of a process (some sequence of commands) appear as a temporary file.

Answer 3

For those looking for a one-liner:

grep xyz abc.txt | while read -r line; do echo "Processing $line"; done

Answer 4

I would suggest using awk instead of grep + something else here.

awk '$0~/xyz/{ //your code goes here}' abc.txt

Answer 5

Without any iteration with the --line-buffered grep option:

your_command | grep --line-buffered "your search"

Real life exemple with a Symfony PHP Framework router debug command ouput, to grep all "api" related routes:

php bin/console d:r | grep --line-buffered "api"

Answer 6

Iterate over the grep results with a while/read loop. Like:

grep pattern filename.txt | while read -r line ; do
    echo "Matched Line:  $line"
    # your code goes here
done

Answer 7

Often the order of the processing does not matter. GNU Parallel is made for this situation:

grep xyz abc.txt | parallel echo do stuff to {}

If you processing is more like:

grep xyz abc.txt | myprogram_reading_from_stdin

and myprogram is slow then you can run:

grep xyz abc.txt | parallel --pipe myprogram_reading_from_stdin